A phase-change material (PCM) is a substance with a high heat of fusion, which, by melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent-heat storage (LHS) units. The phase change herein would be the solid-liquid phase change. Depending on the molecular weight and the type of wax material used, one could tailor the phase change for various temperatures. U.S. Pat. No. 6,939,610 describes phase change materials. This patent is incorporated by reference as if fully set forth herein. PCMs take advantage of the latent heat that can be stored or released from a material over a narrow temperature range. PCMs possess the ability to change their state within a certain temperature range. These materials absorb energy during the heating process as phase change takes place and release energy to the environment in the phase change range during the reverse, that is, the cooling process. Insulation effect reached by the PCM depends on temperature, time, and the type of material employed as phase change material.
Latent-heat storage is one of the most efficient ways of storing thermal energy. Unlike the sensible heat storage method, the latent-heat storage method provides much higher storage density, with a smaller temperature difference between storing and releasing heat. Every material absorbs heat when its temperature is rising constantly. The heat stored in the material is released into the environment through a reverse cooling process. During the cooling process, the material temperature decreases continuously. Comparing the heat absorption during the melting process of a phase change material with those in normal materials, much higher amount of heat is absorbed when a PCM melts. A paraffin-PCM, for example, absorbs approximately 200 kJ/kg of heat when it undergoes a melting process. High amount of heat absorbed by the paraffin in the melting process is released into the surrounding area in a cooling process, which starts at the PCM's crystallization temperature.
During the complete melting process, the temperature of the PCM as well as its surrounding area remains substantially constant. The same is true for the crystallization process; during the entire crystallization process the temperature of the PCM does not change significantly either. The large heat transfer during the melting process as well as the crystallization process without significant temperature change makes PCM interesting as a source of heat storage material in practical applications. When temperature increases, the PCM microcapsules absorb heat and store this energy in the liquefied phase-change materials. When the temperature falls, the PCM microcapsules release this stored heat energy and consequently PCMs solidify.
PCMs can be classified as: (1) organic phase change materials; (2) inorganic phase change materials; and (3) eutectic phase change materials.
Organic PCMs are most often composed of organic materials such as paraffins, fatty acids, and sugar alcohols. For building applications, paraffinic PCMs are the most commonly used for several reasons. First, paraffinic PCMs are straight chain n-alkane hydrocarbon compounds such as n-heptadecane and n-eicosane. Their melting temperature and phase change enthalpy increase with the length of the carbon chain. When the number of carbon atoms in the paraffin molecule is between 13 and 28, the melting temperature falls within a range of approximately 23° to 140° F. (−5° to 60° C.), a temperature range that covers building applications in most climates around the world. In addition, paraffinic PCMs are chemically inert, nontoxic, reliable, and biocompatible. They also show a negligible sub-cooling effect. Fatty acids are represented by the chemical formula CH3(CH2)2nCOOH (e.g., capric acid, lauric acid, and palmitic acid). Fatty acids have storage densities very similar to paraffins, and like paraffins their melting temperatures increase with the length of the molecule. Although chemically stable upon cycling, they tend to react with the environment because they are acidic in nature. Sugar alcohols are a hydrogenated form of a carbohydrate such as D-sorbitol or xylitol, among others. They generally have higher latent heat and density than paraffins and fatty acids. Because they melt at temperatures between 194° and 392° F. (90° and 200° C.), though, they are unsuitable for building applications.
These paraffin-based PCMs are made by physical microencapsulation of the paraffin core in a polymeric shell—the microcapsules act as tiny containers of solids. Generally, microcapsules have walls less than 2 μm in thickness and 20-40 μm in diameter. The microcapsules are produced by depositing a thin polymer coating on core particles. The core contents—the active substance—may be released by friction, by pressure, by diffusion through the polymer wall, by dissolution of the polymer wall coating, or by biodegradation. For example, in their application in textiles, the paraffins are either in solid or liquid state. In order to prevent the paraffin's dissolution while in the liquid state, it is enclosed into small plastic spheres with diameters of only a few micrometers. These microscopic spheres containing PCM are called PCM-microcapsules.
Microcapsule production may be achieved by means of physical or chemical techniques. The use of some techniques has been limited to the high cost of processing, regulatory affairs, and the use of organic solvents, which are concern for health and the environment. Physical methods are mainly spray drying or centrifugal and fluidized bed processes which are inherently not capable of producing microcapsules smaller than 100 □m. Interfacial polymerization techniques are used generally to prepare the microcapsules.
It is clear that PCM microcapsule materials require a physical deposition of a polymeric shell that encases the active material—for example, paraffing—as core. This physical encasement of the core is an expensive process as it requires a chemical in situ polymerization process or another deposition technique, for example, chemical vapor deposition. Moreover, the complete and comprehensive encapsulation of the core by the polymeric shell can interfere with the efficiency of the core material, which really provides the PCM character to the microcapsules.
The present invention addresses the above problems and provides PCMs that are not a classic core-shell structure but wax-based microstructures that are colloidally protected in a casing by polymeric moieties such as PVOH that provides the same functionality by using paraffins with various melt point as core. However, the so-called “encapsulation” in the present invention is not a physical deposition of the polymeric shell on a core, which is what the art teaches.